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Exhaust design discussion

Twisted Steel Performance

Twisted Steel Performance

Anything worth doing is worth overdoing.
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This is something I have read a lot about and I will be crossing this path when I install the new motor so I thought a discussion thread might bring up something I haven't thought of...

It is known that a forced induction engine, turbo in this case, doesn't like a lot of back pressure. Almost every thing I have read says go as big as possible from the turbo back, same size all the way, the more HP the bigger the exhaust should be..

Most know I will use a HE351VE and the no longer sold Banks mannie that moves the turbo to the 2nd cylinder not the front or 3rd cly like the stock mannie. I will have room to fab the down pipe as big as 4 1/2" turbo back..

Now here is what I can't find info about.... my plan is to transition from round to oval just off the turbo down past the frame. Their is more than enough room for 4 1/2" oval to slide past the frame and motor ( I think ) ... So the real question will having the down pipe transition from round to oval then back to round matter ???
 
It will loose some cross sectional surface area in the oval part. How much depends on how much you squish it. How much will it affect it is a unknown at least to me. My guess is you'd have to have both round and squished on a Dyno to see any difference.
 
I would ring up @6.5L and talk to him into making some more headers like he made... This is above and beyond the normal everyone and their brother does turbo or cat back on anything... Even the Duramax has a restricted driver side exhaust manifold to clear some steering shafts. The Banks kit was still a small turbo after all. IMO all it did was clear the smoke a 6.2 NA was known for. Are you leaving restriction in the system with the Banks manifold due to the intended small Banks turbo it was designed to push through?

Anywhere the exhaust comes close to the passenger floor pan should be wrapped, cough cough, coated, cough, so that it doesn't burn the paint.

After dropping a Garrett Ball Bearing turbo on the Dodge Cummins I suggest you could try a Ball Bearing Turbo next. The Ball Bearing saves several shaft horsepower from less friction, less oil heating, and spooled at a low RPM for it's size. You may be able to step the turbo up a size and have the same RPM spool up as a smaller size Journal Bearing turbo. Aka less exhaust restriction needed to drive the Ball Bearing vs. Journal Bearing turbo. Just saying at the turbo's 100,000 RPM a little friction saved goes a long way.
 
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Think "cobrahead" its a proven design used on Cummins and FORD claims some pretty good HP increase numbers using the same design on their PS.

As transition from round to oval and back goes SPIN TECH out in Cali has the market cornered with custom fabrications they even make a 6" oval tube.

I've seen some ricer setups with a large diameter downpipe "more an expansion chamber" into a smaller exhaust system that enhances turbine spool-up.
 
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This is looking like the territory of a fluid dynamics engineer, of which I am not . . .

But even with the disclaimer, I have seen where bigger is not always better. Specifically, with my DI rig, it is common to move up to 4" from turbo to tail as there are efficiency gains, but going to 5" only changes the sound.

From what I have seen in other applications (and suspect that you already know this) a tuned exhaust will help the 6.5 a lot as the pulse waves are all over the place and colliding.

But back to the topic, I have learned that getting a good balance in both the intake *and* exhaust pressures is desirable where the goal is to have them as equal as possible. From what I have seen, Quadstar recognized this. Point is, the 6.5 might just do as good as possible with the common aftermarket downpipe as long as the motor gets an intake to match it.

Another thing toward gains is to limit the exhaust's rotation through the pipe as rotation leads to higher resistance and a longer path to exit.
 
One of FORDS many patents has proven round to oval and back in turns within a cooling system application actually increases flow as opposed to conventional round elbows.

The same concept is used on their turbine elbows to give gains in excess of 20 hp over round tubes & elbows.....

Cummins developed the cobrahead concept many, many decades ago and there a few different versions 4" in/out
 
@Twisted Steel Performance , I have pondered the same question. I do not know the answer. I had looked at going from 4in to dual 3in to get past frame and back to single 4in. I wonder if the guy that built my car may have knowledge on this since he has been doing Headers and exhaust systems for a long time. I will reach out, and post if I can get any info.
 
unless you squish it a bunch I would think the bends in the rest of it to the back would make more difference. I know on bath fans they give a maximum duct length and then you have to subtract "X" amount of feet for each, 90 it adds up fast
 
Plugging and chugging the equation for an oblong pipe of dimensions 3x5-1/8 I get an equivalent round diameter of ~4.1"

de = 1.55 (π b^2 / 4 + a b - b^2)^0.625/(π b + 2 a - 2 b)^0.25
a= 5.125
b=3

Someone can check my math its been a while.

This is a mathematical approximation. It does not take into account flow turbulence, bends, swirls/eddies etc.

I would say it is an improvement over a 3" down pipe. That is IF, the transition and bends are near as smooth as the easy bends of a mandrel bent down pipe.

Like mentioned earlier direction changes are the killer.

My gut tells me it will help off idle spooling a bit as the flowrate would be so low it would be kind of like someone blowing smoke rings from a cigar. Then as flowrate increases the size equivalent of 4.1 over 3" is a ~ 36% size increase that is appreciable so an improvement (as long as transitions and bends are smooth).

My gut tells me the turbo wheel "chops up"/meters the flowrate and averages out the cylinder pulses in the downpipe/exhaust pipe but not in the mannies.

I think it could lower egt not sure how much power it will add.

Probably worth it in your build and more questionable the closer to stock.
 
I don't know just saying there seems to be a fudge factor more than just looking at square inches comparison according to the link I posted. Looking at the chart at square duct 100mm by 200mm is equivalent to 152mm diameter
Comparing just area
square duct 100x200 =20,000 square mm
vs 152mm dia = ~18,145 square mm

If you want the same area its ~159.5 mm dia = ~19,980 square mm

Either way, you want to compare them area calculation vs a de formula I still think it comes out ahead.

3.8 diameter equivalent cross section is a ~26% increase in size over 3" diameter round pipe.

Drive pressure, EGT, MPG, maybe spool response would be the way to measure improvement I would think.
 
Going to be building mine also. I would love to understand the differences in transitions like round to oval, and the cobra head. I’m a half decent fabricator, and being able to make cobra head where needed would be nice.

If you Have access to that flow-bench still....Build and test a couple variations would be fun.
 
If you're just changing the shape .going from round to oval then back to round..it's no different than moving water the same amount can pass through the same area since you're not changing the overall diameter of the pipe.. here's a crude example..I'm posting two pictures of the end of a styrofoam cup.. the first one is its original shape perfectly round... Second one is when I squeeze the sides which make the opposite sides push out farther.. mathematically I'm not changing the overall area of the opening.. so the same volume should be able to pass through it..
Bonus 10 points for the first person who can tell me what's wrong with these pictures..
 

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Stoney said:
If you're just changing the shape .going from round to oval then back to round..it's no different than moving water the same amount can pass through the same area since you're not changing the overall diameter of the pipe.. here's a crude example..I'm posting two pictures of the end of a styrofoam cup.. the first one is its original shape perfectly round... Second one is when I squeeze the sides which make the opposite sides push out farther.. mathematically I'm not changing the overall area of the opening.. so the same volume should be able to pass through it..
Bonus 10 points for the first person who can tell me what's wrong with these pictures..
Click to expand...
No beer in the cups?
 
Haha, i must be too much a kid at heart, i was thinking -you haven’t mixed in the koolaid yet?

The volume of the round- oval- round isn’t hard to figure, it is a question of trying to maintain linear flow to keep out as much turbulence as possible. The rate at which it goes from one shape to the next plays in also. Want to keep the speed up as much as possible. Every time the is a change in direction the air slows.

This is where it really sucks that Nascar is not really stock car racing. And rules eliminating technological advancement is dumb too. True “stock” cars would promote more advancement on things like best flowing exhaust systems. Race car= open loud exhaust. Best flow for performance is a straight pipe coming off he turbo to open air in a foot or so, But not practical.

I have a theory of exhaust gasses flowing faster from vortex generated by the turbo and metered influx of fresh air shortly after the turbo to generate a thermal variation and therefore a pressure differential. That means its a pump. So now you could actually create negative pressure zone tuned to specific engine rpm.

Yes, we really need a fluid dynamics engineer here to explain how much certain things help.

Like manifolds are advantageous because the thick heavy cast iron keeps in more heat to the turbo. Make the exact manifold out of header pipe and leave it uninsulated, you loose heat and therefore power, mpg, etc. retaining the heat also keeps the gasses hotter so it is less dense and will flow faster. Colder exhaust is less dense, so it takes more power to push it out the whole pipe because you have more volume. This is why bigger exhaust pipes isn’t always better.

In race cars that have some exhaust, thinnest header material is used for weight savings. But for a truck where you really don’t care about an extra 20 lbs, building headers or entire exhaust systems out of thick walled pipe can be beneficial by trapping more heat inside the pipe.

Think of using a torch to heat 0.065 tubing to red hot vs 0.120 wall. Takes more than twice as long. Build that header out of schedule 40 pipe and wrap it. That will hold in WAY more heat than normal header will. Plus no worries on cracking and if wrapped header (or crossover pipe) rusting away from it would take how much longer...
 
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